how does the RGB curve work?
Via RGB Curves node (whether Cycles or Compositor) you can adjust contrast, gamma, gain and also offset adjustments. The displayed curve refer to a given color input brightness. The horizontal axis represents the input values, and the vertical axis represents the output values.



Enable C (Composite) to adjust RGB channels simultaneously. Enable R, G, or B if you want to limit the adjustment to a particular color channel.



Also see the manual:


Cycles RGB Curves: https://www.blender.org/manual/render/cycles/nodes/types/color/rgb_curves.html
Composite RGB Curves: https://www.blender.org/manual/compositing/types/color/rgb_curves.html